1. Field of the Invention
The present invention generally relates to a method for manufacturing a laser diode used as a light source in an optical communication system. The invention relates more particularly to a method for manufacturing a diffraction grating, which is an essential process in manufacturing a Distributed Feed-Back Laser Diode (hereinafter, referred to as “DFB-Laser Diode”) or an EML (Electroabsorption Modulated Laser).
2. Description of the Related Art
In general, a laser is an optical element providing strongly-oriented monochromatic light. A semiconductor laser is widely used in optical communication or optical-information processing, etc. The semiconductor laser is preferred because it is smaller than a ruby laser or a gas laser; it uses less power; its laser output can be modulated easily; and, it has a higher output than a He-Ne laser.
The manufacture of a diffraction grating, which provides an excellent optical mode from photon coupling, also has a great influence on the characteristics of a DFB-Laser Diode. In order to obtain an excellent single-mode (low order) spectrum, it is important to obtain a minute and correct grating cycle (A) of the diffraction grating.
A conventional manufacturing technique for the diffraction grating of a DFB-Laser Diode now will be described with reference to FIGS. 1a and 1b. 
First, an InGaAs absorption layer and an InP cap layer are deposited on an InP substrate, then a SiN (or SiO2) film is vapor deposited thereon, and typical photo-etching and wet-etching processes are performed, so that the SiN film pattern 5 is left only at portions where no diffraction grating is to be formed, as shown in FIG. 1a. 
Thereafter, a photoresist for the purpose of manufacturing a diffraction grating is applied on top of the entire structure. Next, light exposure and development processes are performed to form a mask pattern arranged at an interval corresponding to a predetermined grating cycle of the diffraction grating (not shown).
Subsequently, the underlying InP cap layer and InGaAs absorption layer are etched using the photoresist as an etching mask as shown in FIG. 1b. The photoresist pattern and some parts 55 of the SiN film are then removed. This way, no part is etched where the SiN film is present. Only the parts absent of the film are etched, thereby enabling selective manufacturing of a diffraction grating.
However, the conventional method described above has a disadvantage in that when a photoresist for use in manufacturing a diffraction grating is applied on the SiN-film pattern formed in the area where no diffraction grating is to be formed, a “photoresist peeling-off phenomenon” occurs. That is, the photoresist 4 is peeled off at the “A” area in FIG. 1b due to the weakened adhesion of the photoresist. Moreover, there is another problem in that a diffusion phenomenon produced during a subsequent wet-etching process for forming a diffraction grating increases the etching rate in the boundary portions (“B” in FIG. 1b) of the SiN film, thus deteriorating the etching uniformity.